I would not recommend the MEMORY storage engine 

#REASON #1 : No Redundancy

Whether you have a server crash or a normal system shutdown, all the data in the MEMORY table are lost. All you would have is the table structure.

#REASON #2 : Mild Disk I/O

No matter what Storage Engine you choose, the `.frm` of a table is always accessed to check for table existence and availability. This will incur some disk I/O for this check.

Please read past posts on the pros and cons of the MEMORY Storage Engine

- `May 22, 2011` : http://dba.stackexchange.com/questions/2868/i-am-using-the-memory-storage-engine-but-mysql-still-writes-to-my-disk-why/2876#2876
- `Sep 26, 2011` : http://dba.stackexchange.com/questions/6156/is-it-feasible-to-have-mysql-in-memory-storage-engine-utilize-512-gb-of-ram/6176#6176
- `Jan 17, 2012` : http://dba.stackexchange.com/questions/10806/mysql-memory-table-getting-many-locks/10821#10821
- `Jan 20, 2012` : http://dba.stackexchange.com/questions/11912/how-much-memory-will-a-memory-table-take-up/11917#11917

#RECOMMENDATION

Given the two reasons for not using the MEMORY Storage Engine, I would recommend the MyISAM Storage Engine over using MEMORY or InnoDB. Why?

Looking back at Reason #1, you can have everything in RAM and have data redundancy on disk if you create the table as follows:

STEP 01) Create the table like this:

    CREATE TABLE blacklist
    (
        url VARCHAR(20),
        dt DATETIME,
        PRIMARY KEY (url)
    ) ENGINE=MyISAM ROW_FORMAT=Fixed;

STEP 02) Create a dedicated 16MB MyISAM cache for that table:

    cd /var/lib/mysql
    echo "SET GLOBAL blacklist_keycache.key_buffer_size = 1024 * 1024 * 16;" > init-file.sql
    echo "CACHE INDEX blacklist INTO blacklist_keycache; >> init-file.sql
    echo "LOAD INDEX INTO CACHE blacklist_keycache; >> init-file.sql

STEP 03) Add this to /etc/my.cnf

    [mysqld]
    init-file=/var/lib/mysql/init-file/sql

STEP 04) Restart MySQL

    service mysql restart

That's it.

Going forward, every reload of the table will populate the dedicated key cache. Please note the `ROW_FORMAT=Fixed clause`. [**What that does is speed up character search 20-25% (I wrote about this before)**][1].

Why not use InnoDB?

- The data and index pages would have the data twice in RAM.
- Accessing an InnoDB table introduces additional mild disk I/O via data dictionary checking.
- Index pages to rotate out is the InnoDB Buffer Pool is too small. Contrariwise, an InnoDB Buffer Pool too big wastes RAM.

Using MyISAM, the data remains on disk but is exclusively accessed from the dedicate key cache.


  [1]: http://dba.stackexchange.com/a/4589/877